The invention relates to a laser nozzle usable in laser beam cutting, having an internal movable element comprising a skirt that allows the cutting gas to be funneled into the cutting kerf, and furthermore being more able to withstand industrial use and easier to implement industrially.
Laser beam cutting requires the use of a nozzle, generally made of copper, that channels the gas and allows the laser beam to pass.
These nozzles typically have outlet orifice diameters comprised between 0.5 and 3 mm for a working distance comprised between 0.6 and 2 mm.
In order to enable cutting, it is necessary to use high pressures, in general several bar, in the focusing head in order to allow the gas to penetrate into the kerf and flushes out molten metal.
However, a large percentage of the gas used, typically between 50 and 90%, does not take part in the cutting process, i.e. in the expulsion of molten metal, because it is lost to the sides of the cutting kerf.
These gas losses are in fact due to the enormous difference between the flow cross-sectional area of the nozzle orifice and the size of the focal spot. Thus, by way of indication, the flow cross-sectional area of a nozzle with an outlet orifice of diameter equal to 1.5 mm is 25 times larger than the cross-sectional area of the focal spot created by the beam passing through this nozzle.
However, if an insufficient amount of gas penetrates into the kerf, cutting defects will be observed, in particular attached burrs and/or oxidation marks.
Attempting to solve this problem by decreasing the diameter of the orifice of the nozzle is not ideal because the risk is then taken that the laser beam will strike and deteriorate the interior of the nozzle. Decreasing the diameter of the orifice of the nozzle also decreases cutting quality and/or performance.
There are moreover a number of documents proposing various solutions that attempt to encourage gas to penetrate into the kerf, documents EP-A-1669159, JP-A-62006790, JP-A-61037393, JP-A-63108992, JP-A-63040695 and U.S Pat. No. 4,031,351 for example.
However, none of these solutions is truly ideal because they either have an architecture that is complicated to implement, are bulky relative to conventional nozzles and/or of limited effectiveness.
Document U.S. Pat. No. 4,031,351 in particular discloses a laser cutting nozzle comprising a movable element the end of which is pressed by a spring against the surface of the part to be cut in order to encourage the injection of the cutting gas into the kerf. Document DE 198 53 735 C1 also discloses a nozzle having a movable element, in which nozzle a spring tends to move the movable element in the direction of the part.
The major drawback of this solution resides in the fact that the force exerted by the spring in the direction of the sheet, added to the pressure of the cutting gas, causes the movable element to exert a substantial force on the sheet to be cut. There is therefore a risk that the sheet will be deformed, scratched or even dragged, as in general the sheet is simply placed on the table of the industrial cutting machine.
To remedy this drawback, it has been proposed in French patent application No. 1 154 224, filed 16 May 2011, to arrange a movable element in the body of a laser nozzle. This movable element is able to move axially in said body, in the direction of the surface of the sheet to be cut, under the effect of a gaseous pressure. As a result the movable element moves toward the upper surface of the sheet to be cut until it makes contact therewith. The movable element thus forms a skirt that channels and concentrates the cutting gas into the kerf, thereby forcing the gas to penetrate into the kerf and improving its effectiveness.
However, this solution continues to pose certain problems, especially in the context of industrial use.
Thus, industrial laser cutting machines employ capacitive distance sensing systems to ensure that the focusing head is moved at a constant distance above the sheet to be cut.
These systems use a capacitive effect to detect small variations in distance between two conductive elements forming a capacitor. The distance separating the two conductive elements is determined by measuring the electrical capacitance of this capacitor, which especially depends on the dielectric permittivity of the medium separating them.
Use of such a sensor system is especially described in document U.S. Pat. No. 6,130,405.
In a cutting machine equipped with a conventional laser nozzle, generally formed from an electrically conductive material such as copper, the capacitive sensor measures the electrical capacitance between the sheet and the flat end surface of the nozzle i.e. the surface located facing the sheet. The capacitive sensor is electrically connected to means for controlling the movement of the focusing head so as to adjust the height of the head if the electrical capacitance measured varies or stop the movement of the head if the nozzle and the sheet make contact.
This capacitive type of sensing system makes it possible to guarantee a cutting performance that is constant in terms of speed and cutting quality, by maintaining the focal point of the laser beam in a constant position relative to the surface of the sheet. It also makes it possible to stop the machine in the case where obstacles are present on the sheet.
It is therefore essential not to disrupt its operation.
However, the use of a laser nozzle having a movable element such as described in French patent application No. 1 154 224 is difficultly compatible with such a system.
Specifically, the movable element of the nozzle forms a skirt that makes contact with the sheet to be cut. In order to guarantee that this movable element is able to resist the heat given off by the cutting process and the spatter of molten metal, it may be formed from a metallic material such as copper, brass or analogous. However, the metal movable element then makes contact both with the sheet, i.e. it is at the same electrical potential as the latter, and with the internal walls of the nozzle body, generally itself also formed from an electrically conductive material. It is therefore necessary to deactivate the capacitive sensor in order to prevent the cutting machine from malfunctioning.
One solution that would permit operation of the capacitive sensor of the machine would be to use a movable element formed from an electrically insulating material. However, this solution is not ideal because electrically insulating materials are in general not very resistant to the large amount of heat given off by the cutting process and to spatter of molten metal and/or thermal shocks.
The problem to be addressed is therefore how to mitigate all or some of the aforementioned drawbacks, especially by providing a laser nozzle that, relative to existing solutions, has a greatly improved robustness and lifetime and is much easier to implement industrially, and that does not disrupt, or clearly less so than the prior art, the operation of capacitive distance sensing systems with which industrial cutting machines are equipped.